Method for Programming of Memory Cells, in Particular of the Flash Type, and Corresponding Programming Architecture

ABSTRACT

A method is described for programming memory cells, in particular of the Flash type. In accordance with the method, a verification is performed with a first parallelism (M) in which a reading is carried out for determining the state of a group of memory cells, a determination is performed of a programming parallelism (np), based on the results of the verification, and a real programming of the memory cells carried out with the programming parallelism (np). An architecture is also described for programming memory cells in particular of the Flash type.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/636,382, filed on Dec. 8, 2006, which claims priority from ItalianPatent Application No. MI2005A 002350 filed Dec. 9, 2005, the disclosureof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a method for programming memory cellsof the Flash type.

The invention also relates to an architecture for programming memorycells, in particular of the Flash type.

The invention particularly, but not exclusively, relates to a memorydevice of the Flash type suitable for being used in compact disk (CD)players, video cameras, cell phones, and the like, and the followingdescription is made with reference to this field of application forsimplifying its illustration only.

2. Description of Related Art

As it is well known, Flash memories are non volatile memories, able tomaintain the information also in the absence of power supply andorganized so as to be erasable by sector.

The information contained in a common Flash memory cell is of the binarytype, a first high logic value or bit ‘1’ corresponding to one erasedstate and a second low logic value or bit ‘0’ identifying a programmedstate.

The programming operation can be conducted with a parallelism of 16 bits(according to the so called word mode) or 64 bits (according to the socalled page mode), while the erasing operation generally always relatesto an entire sector of the memory, whose current density varies from 0.5to 2 MBits. These operations are managed by a microcontroller embeddedin the Flash memory device that executes suitable programming anderasing algorithms contained in a ROM memory, in turn included in theFlash memory device.

The erasing time (like the programming time) should meet a specificationthat, in current memories, is of about 0.7 sec and that, in the future,will tend to decrease due to the increase of the speed of theapplications whereon the same will be employed, such as for example inthe use inside CD players, video cameras and cell phones, etc.

FIG. 1 shows the typical current-voltage [I-V] characteristics of twomemory cells of the Flash type, respectively in the erased state (bit1)-curve A, and in the programmed state (bit 0)-curve B. As it can benoted in such figure, the I-V characteristics increase starting from aminimum voltage value, respectively VTHEV for an erased cell and VTHPVfor a programmed cell.

It is also known that an erasing operation of a Flash memory comprisesthree distinct steps: a pre-programming step (or ALL0); a real erasingstep (or ERASE); and a soft-programming step (or SOFTP).

The ALL0 step comprises the pre-programming of an entire sector of theFlash memory to be erased so that all the Flash cells of this sector areunder the same initial conditions as regards their threshold voltagevalue. This pre-programming of the cells of the sector usually occurswith a parallelism of 16 bits (per word).

The ERASE step comprises the erasing of the entire sector and itsresolution thus depends on the density of the sector itself. Thisresolution is substantially the minimum memory portion that can beerased, which has dimensions ranging from 0.5 Mbits to 2 Mbits in theconsidered applications.

The SOFT Programming step has the aim of recovering those cells whosethreshold voltage, at the end of the ERASE step, is lower than a minimumvoltage value of the distribution of the erased cells (indicated withVTHDV). This SOFT Programming step has a typical parallelism of 16 bits.

The distributions of the threshold voltages of the cells of a sector ofa Flash memory subjected to an erasing operation—after each one of theabove-indicated steps—are shown in FIG. 2, respectively indicated withDALL0, DERASE and DSOFTP. The area C of the distribution DSOFTPgraphically shows the portion of Flash cells whose threshold voltage islower than the minimum voltage value VTHDV and that should thus undergosoft-programming.

The continuous scaling of the technological processes, with theconsequent reduction of the dimension of the single Flash cells, hasimplied an increase of the unhomogeneity between the cells of a sector,for example between a cell on board and a cell in the center, causing ahigher and higher spread in the distribution of the threshold voltagesof the erased bits as highlighted in FIG. 3, the arrow F indicating atechnological scaling from 0.35 um to 0.09 um.

FIG. 3 highlights how this spread has a direct impact on the need ofsoft-programming a distribution, having passed from distributions whichdid not need any soft-programming (like the distribution D correspondingto the case of the technology with 0.35 um) to distributions which areto be almost totally soft-programmed (like the distribution Ecorresponding to the case of the most recent technology with 0.09 um).

For the distributions of the most recent technologies, moreover, theFlash cells whose threshold voltage is distant from the minimum valueVTHDV being numerous, the same need soft-programming ramps with finalvoltage differences .DELTA.V much higher than those of less recenttechnologies, reaching almost 5V in the case of the technologies with0.09 um.

These two limitations make the soft-programming step, having much lowerparallelism than that of the ERASE step, represent about 70% of thetotal erase time and thus it should be calibrated with care forrespecting the specifications requested for a given Flash memory.

In particular, the erase time Terase is given by the sum of the times ofthe ALL0 (T_(ALL0)), ERASE (T_(ERASE)) and SOFT Programming (T_(SOFTP))steps:

Terase=Σ(T _(ALL0) ,T _(ERASE) ,T _(SOFTP))

As already hinted at, the duration T_(ALL0) of the ALL0 step depends onthe density of the sector of the Flash memory to be erased, on theduration of the programming and on the adopted parallelism, i.e. on thenumber of bits that will be simultaneously programmed (16 bits in theword mode, 64 bits in the page mode).

For example, considering a sector with density equal to 0.5 Mbit, aduration of the programming equal to 5 usec and a parallelism of 16bits, the duration of the ALL0 step will be:

$\begin{matrix}{T_{{ALL}\; 0} = {( {{density}\text{/}{parallelism}} )*{duration}\mspace{14mu} {programming}}} \\{= {{{( {500000/16} )*5*10^{- 6}} \sim} = {160\mspace{14mu} {ms}}}}\end{matrix}$

The duration of the ERASE step instead depends on the duration of theerase pulse, on the overall voltage difference .DELTA.V to be appliedand on the value of the single step of voltage (step_voltage) and it isthus equal to:

T _(ERASE)=duration_erase_pulse*(ΔV/step_voltage)

Considering, for example, an erase pulse of duration 3 msec, an overallvoltage difference ΔV of 5V and a value of the single step of voltage(step_voltage) of 125 mV it results:

T _(EASE)=3*10e⁻³(5/0.125)=120 msec

Finally, the duration of the SOFT Programming step depends, as in thecase of the ALL0 step, on the density of the sector, on the duration ofthe programming, on the parallelism adopted and, finally, on the stepsof voltage which allow to realize a programming ramp with apredetermined final voltage difference ΔV.

For example, if also in this case a sector of density equal to 0.5 Mbit,a programming duration equal to 5 usec, a parallelism of 16 bits, afinal voltage difference ΔV of 3V with steps of 375 mV are considered,the duration of the SOFTP step will be:

T _(SOFTP)=(density/parallelism)*programmingduration*(ΔV/step_voltage)=(500000/16)*5*10e⁻⁶*(3/0.375)=1250 msec

Therefore, the overall time of the erase step, in the above indicatedhypotheses, will be equal to:

Terase=Σ(T _(ALL0) ,T _(ERASE) ,T _(SOFTP))˜=160 msec+120 msec+1250msec˜=1.5 seq

Time which, as it is easy to be verified, is widely out of thespecifications of several applications.

Moreover, it is immediate to verify that the duration of the SOFTProgramming step has a predominant impact on the total erase time, equalto about 70% as indicated above.

Also in the case in which the parallelism adopted were of 64 bits therewould result:

T _(ALL0)=(500000/64)*5*10e⁻⁶˜=40 msec

T_(ERASE)=120 msec (does not depend on the adopted parallelism)

T _(SOFTP)=(500000/64)*5*10e⁻⁶*(3/0.375)˜=320 msec

The overall time of the erase step in this scenario would be thus equalto:

Terase=Σ(T _(ALL0) ,T _(ERASE) ,T _(SOFTP))˜=40 msec+120 msec+320msec˜=500 msec

It is thus evident that the reduction of the erase time is obtained byadopting a high parallelism and that therefore the reduction of thesoft-programming time (which, as above said, represents 70% of the totalerase time) is only a problem of parallelism.

The known erase methods thus provide, already in the design phase, ahigh parallelism of the soft-programming step. In this case, thesoft-programming architecture associated with the Flash memory mustnecessarily comprise a charge pump designed for supplying the Flashcells with programming currents taking into consideration the “worstcase”, i.e., that in which the soft-programming step occurs with themaximum parallelism, the current to be supplied growing with theparallelism.

For example, if a Flash cell absorbs, during the soft-programming step,a current Icell, the charge pump will have to supply a current valueIcharge_pump given by the following relation:

Icharge_pump=Icell*parallelism

Taking then into consideration a value of this current Icell equal toabout 100 uA, the current that the charge pump must supply depends onthe parallelism and it is equal to:

parallelism 16 bits: Icharge_pump=100 uA*16 bit=1.6 MA

parallelism 32 bits: Icharge_pump=100 uA*32 bit=3.2 mA

parallelism 64 bits: Icharge_pump=100 uA*64 bit=6.4 mA

The most evident disadvantages of this approach result in a growingoccupation in terms of silicon area by the charge pump when the chosenparallelism grows and a worse tracking on the process, sincetechnologically more mature processes and thus Flash cells being lessand less demanding in terms of current would no more justify the designchoices aimed at increasing the maximum current which can be supplied bythe charge pump.

If, in the example of parallelism 64 bits, the current Icell absorbed bythe cell decreased from 100 uA to 60 uA it would be enough to have:

Icharge_pump=60 uA*64 bit=3.8 mA

This value is considerably lower than 6.4 mA which was the design targetfor the charge pump of the soft-programming architecture based on theknown method. This, therefore, is over-dimensioned with consequent wasteof resources.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a method forprogramming memory cells. The method makes the parallelism of theprogramming step, in particular of soft-programming, adaptive so that itis a function of the current absorbed by the Flash cell during theprogramming itself, considering the maximum current which can besupplied by the charge pump fixed as per the design constant.

In an embodiment, the method comprises the following steps: a verifystep with a first parallelism in which a reading is carried out fordetermining the state of a group of memory cells; a determination stepof a programming parallelism, based on the results of the verify step;and a real programming step of the memory cells carried out with theprogramming parallelism.

A further embodiment of the invention is directed to a programmingarchitecture of at least one array of memory cells, in particular of theFlash type, of the type comprising at least one microcontroller suitablefor carrying out a programming operation of the cells by using at leastone charge pump, at least one verify block connected to the array ofmemory cells and at least one programming block connected to themicrocontroller, the verify block carrying out a verification of stateof the array of memory cells with a first parallelism and theprogramming block generating a control flag for the regulation of aprogramming parallelism with which this microcontroller carries out theprogramming operation.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be acquired by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIG. 1 shows the pattern of the current-voltage [I-V] characteristics ofFlash cells in two different states, respectively programmed and erased;

FIG. 2 shows the distribution of the threshold voltages of Flash cellsafter various steps of an erasing method realized according to the priorart;

FIG. 3 shows the distribution of the bit threshold voltages after asoft-programming step realized according to the prior art depending onthe technological scaling;

FIG. 4 schematically shows a programming architecture of memory cellsrealized according to the invention;

FIG. 5 shows a situation being statistically likely after an erasingstep of the state of Flash cells to be soft-programmed with the methodaccording to the present invention; and

FIG. 6 shows the threshold voltage distribution of Flash cells to besoft-programmed with the method according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention starts from the consideration that, by maintainingthe maximum current which can be supplied by the charge pumpIcharge_pump constant, it is possible to realize an adaptive parallelismbeing only function of the Icell current absorbed by a Flash cell duringa programming, according to the relation:

parallelism=Icharge_pump/Icell

The considerations made about the soft-programming step, in particularthe impact of the soft-programming time on the erase time of a Flashmemory, have allowed to identify, in the parallelism of thissoft-programming step, one of the main problems of the methods proposedby the prior art. In reality, the proposed adaptive solution can beapplied to a generic programming operation, and it is particularlyuseful in the case of a soft-programming of Flash memory cells.

Advantageously, thus, the present invention proposes a programmingmethod comprising the following steps: a verify step according to afirst parallelism M in which a reading is carried out for determiningthe state, in terms of threshold voltage, of a group of M memory cells,in particular of the Flash type (M bits, with M=16, 32, 64, . . . ); adetermination step of a programming parallelism np, based on the resultsof this first verify step; and a real programming step carried outaccording to the programming parallelism np determined in thedetermination step.

In particular, the verify step determines a number (bit_to_pr) of memorycells, among those verified, which must undergo the programming step.Advantageously, this number bit_to_pr is then stored and used in thenext step for determining the parallelism np of the last programmingstep.

In this way, advantageously according to an embodiment of the invention,where a maximum current Icharge_pump supplied by a charge pump of theprogramming architecture of the memory cells having been fixed, theprogramming step is carried out with a maximum parallelism until thenumber bit_to_pr is lower or equal to the driving capability of the pumpitself, thus obtaining an adaptive programming.

An architecture for programming of memory cells, in particular of theFlash type, realized according to the present invention is schematicallyshown in FIG. 4, globally indicated with 10.

The architecture 10 is connected, by means of a first input bus B1 withM bits, to an array 1 of Flash cells and comprises a verify block 2 anda programming block 3.

In particular, the verify block 2 is connected, by means of the firstbus B1, to the array 1 of Flash cells and, by means of a second bus B2with M bits, to the programming block 3.

Moreover, advantageously according to an embodiment of the invention,the programming block 3 comprises a counter 4 with M bits connected atthe input, by means of the second bus B2, to the verify block 2 and atthe output to a combinatorial portion 5. The counter 4 supplies thecombinatorial portion 5 with the number bit_to_pr of memory cellsneeding to be programmed.

It is to be noted that the value M of the counter 4 is directly relatedto the maximum parallelism used during the verify operation, i.e. thereading operation of the state of the memory cells in terms of thresholdvoltage by the verify block 2, indicated hereafter as first parallelismM.

The programming block 3 also comprises a register 6 with n-bitsindicative of the driving capability of the charge pump comprised intothe Flash memory. The register 6 is a memory element whose dimension canbe trimmed being a function of the design of the adopted charge pump andof the state of evolution of the technology with which the memory isrealized and the value n, hereafter indicated also as secondparallelism, is identified by the relation:

n=Icharge_pump/Icell

Obviously, according to the first parallelism M adopted in the verifyoperation and to the calculation formula of the second parallelism n,i.e. of the dimension of the register 6 above indicated, n can take avalue comprised between:

1<=n<=M

Advantageously according to an embodiment of the invention, anattribution of a set of values is then provided for the secondparallelism n. The adopted charge pump being identical, these values area mirror of the technological evolution of the process, wheretechnologically more mature processes will correspond to n highervalues.

The combinatorial portion 5 receives at the input and compares thenumber of bits to be programmed, bit_to_pr, and the second parallelism nstored in the register 6, generating a control flag PR_PAGE.

In particular, this control flag PR_PAGE is placed at ‘1’ in the case inwhich:

bit_to_pr<=n

In this case, a microcontroller 7 comprised in the Flash memoryconnected to the architecture 10 and connected at the output to thecombinatorial portion 5 carries out the programming operation with amaximum programming parallelism np, i.e. equal to the first parallelismM (exactly because the driving capability of the pump is higher than thenumber of bits to be soft programmed): np=M.

In the contrary case, the control flag PR_PAGE is placed at ‘0’ and themicrocontroller 7 carries out the operation of soft programming with aminimum programming parallelism np (i.e. the second parallelism nensured by the charge pump): np=n.

As it will be seen more clearly hereafter in the description, it is alsopossible to use a programming parallelism of the dichotomic typeaccording to the condition:

if bit_to_pr>n→np=M/2

and so on for the successive comparisons.

Advantageously according to an embodiment of the invention, thearchitecture 10 thus allows to implement a method for programming memorycells of the adaptive type comprising the steps of: verify with a firstparallelism M of the state of a group of cells contained in a Flashmemory; determination of a number (bit_to_pr) of Flash cells among thoseverified which must undergo a programming operation; storage of a secondparallelism n indicative of the driving capability of the charge pumpcomprised into the Flash memory according to the relation:

n=Icharge_pump/Icell

wherein:

Icharge_pump the maximum current which can be supplied by the chargepump, and

Icell the current absorbed by a memory cell during a programming,

comparison between the number of bits to be programmed, bit_to_pr, andthe second parallelism n; generation of a control flag PR_PAGE; andexecution of the real programming operation on the basis of the controlflag PR_PAGE.

In particular, the programming method according to an embodiment of theinvention provides that this generation step of the control flag PR_PAGEmeets the following conditions:

PR_PAGE=1 if bit_to_pr<=n

In this case, the programming operation is carried out with aprogramming parallelism np corresponding to the first parallelism M(maximum parallelism).

PR_PAGE=0 if bit_to_pr>n

In this case, the programming operation is carried out with aprogramming parallelism np corresponding to the second parallelism n(minimum parallelism).

It is to be noted that the choice made according to an embodiment of theinvention of realizing a programming method, in particular ofsoft-programming, of the adaptive type is based on important statisticconsiderations.

In particular, making reference to the architecture 10 shown in FIG. 4and supposing a parallelism in verify equal to M=64 bits (verify perpage, 4 words of 16 bit), a probably true condition for the state of thememory cells of the array 1 after an ERASE operation is shown in FIG. 5,where the array 1 has been suitably divided into four segments S1-S4.

In particular, each ‘0’ represents a cell which does not need to beprogrammed, while the ‘1’ refers to those cells whose threshold voltageis lower than the minimum value VTHDV and which thus are to beprogrammed.

Similarly, making particular reference to a memory of the Flash type andto its erasing, FIG. 6 shows the distribution of the Flash cells of asame page, after a verify operation, according to the threshold voltage,the ‘0’ representing cells which do not have to be soft-programmed andthe ‘1’ cells which are to be soft-programmed. In particular, a firstportion P1 of cells to be soft-programmed and a second portion P2 ofcells not to be soft-programmed are identified.

Although, as previously said, the population of bits to besoft-programmed after the ERASE step is very consistent, FIG. 6highlights how the probability that, in each verified page (composed of4 words), all the bits are to be soft-programmed is close to zero. Thiscondition, in fact, would be represented by a distribution having themaximum coinciding with the value VTHDV, the case being clearly absurdexactly because the ERASE operation is carried out by verifying that thethreshold voltage has, as limit, the minimum erase value VTHEV (aspreviously shown with reference to FIG. 2).

These considerations allow to soft program with a maximum parallelism Min most cases, limiting the programming parallelism np, in particular tothe value n linked to the charge pump, in the uncommon cases in which,inside a same page, the number of bits to be soft programmed (bit_to_pr)is greater than the driving capability of the adopted pump.

For these uncommon cases, the programming method proposed adopts adichotomic approach. In particular, a page which, given the number ofbits to be soft-programmed bit_to pr being higher than the drivingcapability of the adopted pump, cannot be soft-programmed with maximumparallelism M=64, is programmed in two steps of np1=32 bits each, or infour steps of np2=16 bits, by applying the above indicated rulebit_to_pr<n.

A possible alternative to the dichotomic approach above described, thesecond parallelism n (identifying the maximum parallelism which can beadopted on the basis of the driving capability of the charge pump)having been chosen, comprises programming a whole page with a number ofpulses equal to:

bit_to_or/n=IMP, equal to an integer number of pulses

For example, if after a page verify there were 38 bits to be programmedand if n were 26, the number of pulses to be given would be equal totwo, i.e. the managing logic of the above described solution wouldprogram the first 26 bits in parallel and the remaining 12 at thesuccessive pulse.

The parameter n, identifying the programming or soft-programmingparallelism, has been set during an EWS phase of the Flash memory deviceand thus, potentially, can vary from lot to lot as from slice to slice.In this case, the architecture 10 also comprises a portion for theloading of this value n in the register 6, not shown in the figure sinceconventional.

It is also possible to vary, in an embedded way, the parameter n duringthe life of the device and according to the current absorbed by theFlash cells during the programming and/or soft-programming steps. Inthis case, the method provides a comparison step of the current absorbedby the Flash cells with that supplied by an inner current reference anda modify step of the parameter n according to the result of thiscomparison. This allows a greater flexibility as regards the use of thememory device under the various operation conditions.

It is to be noted that it is possible to use the above shown programmingmethod also for the ALL0 step previously described in relation to theprior art.

In fact, the erase operation can occur also on sectors whereon a patterndifferent from ALL1 has been written, i.e. on a sector which comprises agroup of Flash cells which are not all at the value ‘1’, i.e. such thatthe bits to be pre-programmed in the ALL0 step do not coincide with thetotality. In this case, it is possible to realize the pre-programmingoperation of the ALL0 step with the steps seen for the programmingmethod according to an embodiment of the invention, i.e. with the stepsof: verify of a page; counting of the bits to be programmed; choice ofthe programming parallelism np to be adopted; real programming.

Moreover, it is possible to trace to the same principles also a factoryprogramming operation (like the Double Word Program and Tetra WordProgram). In particular, a factory programming operation is usuallycarried out with the help of a very high external voltage (about 12V).

Advantageously, by using the principles of the programming methodaccording to an embodiment of the invention, it will be possible tocarry out the same factory programming operation with a low voltage,having however a small increase of the execution time. This compromisecan be desirable in many cases, since the use of programmers managinghigh voltage is rather onerous in economical terms, and the currenttrend is that of lowering the rated voltage.

Advantageously, the programming method according to an embodiment of theinvention obtains a dynamic parallelism which exploits the drivingcapability of the used charge pump at the maximum, no matter what thepump is, and, in the meantime, takes into account the maturity attainedby the process.

Advantageously according to an embodiment of the invention, in fact, inthe case of a possible 40% reduction of the Icell current absorbed bythe Flash cell in the soft-programming step, the parallelism of the samesoft-programming step increases by the same extent, thus obtaining agreat reduction of the erase time of the Flash memory.

On the contrary, the methods proposed by the prior art tend to solve theproblem with the theory of the “worst case”, wasting resources in thedesign of a charge pump able to sustain a high parallelism.

In conclusion, the programming method according to an embodiment of theinvention, and the corresponding architecture, allow to annul the wasteof resources employed in the design of charge pumps able to sustain highparallelisms, waste which still affects the architectures designedaccording to the prior art. Advantageously, in this way, resources interms of design time and silicon area are obtained by realizing also anexcellent tracking on the process.

Moreover, the programming method according to an embodiment of theinvention shows erase times which can be compared with the methods ofthe prior art which use the “expensive” approach of the “worst case”,but at practically null costs.

Although preferred embodiments of the device of the present inventionhave been illustrated in the accompanying Drawings and described in theforegoing Detailed Description, it will be understood that the inventionis not limited to the embodiments disclosed, but is capable of numerousrearrangements, modifications and substitutions without departing fromthe spirit of the invention as set forth and defined by the followingclaims.

1. A method for programming memory cells, in particular of the Flashtype, comprising the following steps: a verify step with a firstparallelism in which a reading is carried out for determining the stateof a group of memory cells; a determination step of a programmingparallelism, based on the results of the verify step; and a realprogramming step of the memory cells carried out with the determinedprogramming parallelism.
 2. The programming method of claim 1, whereinthe verify step determines a number of memory cells among those verifiedwhich should undergo the real programming step and in that it furthercomprises a storage step of the number of memory cells to be programmed.3. The programming method of claim 2, further comprising the steps of:storage of a second parallelism indicative of a driving capability of acharge pump for the generation of a programming voltage of the cellsaccording to the relation:n=Icharge_pump/Icell wherein: Icharge_pump is a maximum value of thecurrent which can be supplied by the charge pump, and Icell is a valueof current absorbed by a memory cell during a programming; comparisonbetween the number of memory cells to be programmed and the secondparallelism, and generation of a control flag according to therelations: PR_PAGE=1 if the number of memory cells to be programmed islower or equal to the second parallelism (bit_to_pr<=n); and PR_PAGE=0if the number of memory cells to be programmed is higher than the secondparallelism (bit_to_pr>n).
 4. The programming method of claim 3, whereinthe determination step of the programming parallelism comprises thefollowing steps: if PR_PAGE=1, attribution to the programmingparallelism of a value corresponding to a value of maximum parallelism(np=M); and if PR_PAGE=0 attribution to the programming parallelism of avalue corresponding to a value of minimum parallelism (np=n).
 5. Theprogramming method of claim 4, wherein the value of maximum parallelismcorresponds to the first parallelism.
 6. The programming method of claim4, wherein the value of minimum parallelism corresponds to the secondparallelism.
 7. The programming method of claim 3, further comprising adetermination step of a number of programming pulses according to therelation:bit_to_pr/n=number of pulses, the number of pulses being an integernumber and the programming step comprising the number of pulses.
 8. Theprogramming method of claim 3, wherein the determination step of theprogramming parallelism is of the dichotomic type according to therelation: if the number of memory cells to be programmed is higher thanthe second parallelism (bit_to_pr>n), attribution to the programmingparallelism of a value corresponding to half of the value of maximumparallelism (np=M/2); this relation being consecutively applied up tothe exhaustion of the cells to be programmed.
 9. The programming methodof claim 3, further comprising a setting step of the second parallelism.10. The programming method of claim 3, further comprising a calculationstep of the second parallelism by means of comparison of a value of thecurrent absorbed by the memory cells during a programming and/orsoft-programming step with a value of reference current.
 11. A methodfor programming memory cells comprising making a parallelism of theprogramming step, in particular of soft-programming, adaptive so that itis a function of the current absorbed by a Flash memory cell during theprogramming itself, and considering a maximum current which can besupplied by a charge pump which has been fixed as per a design constant.